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Abstract:

The present invention relates to a process for reducing cold start
emissions in an exhaust gas stream by contacting the exhaust stream with
a combination of molecular sieves comprising (1) a small pore crystalline
molecular sieve or mixture of molecular sieves having pores no larger
than 8 membered rings selected from the group consisting of SSZ-13,
SSZ-16, SSZ-36, SSZ-39, SSZ-50, SSZ-52 and SSZ-73 and (2) a medium-large
pore crystalline molecular sieve having pores at least as large as 10
membered rings selected from the group consisting of SSZ-26, SSZ-33,
SSZ-64, zeolite Beta, CIT-1, CIT-6 and ITQ-4.

Claims:

1. A process for treating a cold-start engine exhaust gas stream
containing hydrocarbons and other pollutants comprising flowing said
engine exhaust gas stream over a combination of molecular sieves which
preferentially adsorbs the hydrocarbons over water to provide a first
exhaust stream, and flowing the first exhaust gas stream over a catalyst
to convert any residual hydrocarbons and other pollutants contained in
the first exhaust gas stream to innocuous products and provide a treated
exhaust stream and discharging the treated exhaust stream into the
atmosphere, the combination of molecular sieves comprising only an oxide
(1) having a small pore crystalline molecular sieve selected from the
group consisting of SSZ-13, SSZ-16, SSZ-36, SSZ-39, SSZ-50, SSZ-52,
SSZ-73 and combinations thereof, and having a mole ratio of at least 10
of (a) an oxide of a first tetravalent element to (b) an oxide of a
trivalent element, a pentavalent element, or a second tetravalent element
which is different from said first tetravalent element or mixture thereof
and an oxide (2) having a crystalline molecular sieve having pores at
least as large as 12 membered rings and selected from the group
consisting of SSZ-64, zeolite Beta, CIT-1, CIT-6, ITQ-4, and combinations
thereof, and having a mole ratio of at least 10 of (a) an oxide of a
first tetravalent element to (b) an oxide of a trivalent element, a
pentavalent element, or a second tetravalent element which is different
from said first tetravalent element or mixture thereof.

6. The process of claim 2, wherein the engine is an internal combustion
engine.

7. The process of claim 6, wherein the internal combustion engine is an
automobile engine.

8. The process of claim 2, wherein the engine is fueled by a
hydrocarbonaceous fuel.

9. The process of claim 2, wherein the molecular sieve has deposited on
it a metal selected from the group consisting of platinum, palladium,
rhodium, ruthenium, and mixtures thereof.

10. The process of claim 9, wherein the metal is platinum.

11. The process of claim 9, wherein the metal is palladium.

12. The process of claim 9, wherein the metal is a mixture of platinum
and palladium.

13. The process of claim 1, wherein the cold-start engine exhaust gas
stream is contacted with the small pore crystalline molecular sieve prior
to contacting the crystalline molecular sieve having pores at least as
large as 12 membered rings.

14. A process for treating a cold-start engine exhaust gas stream
containing hydrocarbons and other pollutants comprising flowing said
engine exhaust gas stream over a combination of molecular sieves which
preferentially adsorbs the hydrocarbons over water to provide a first
exhaust stream, and flowing the first exhaust gas stream over a catalyst
to convert any residual hydrocarbons and other pollutants contained in
the first exhaust gas stream to innocuous products and provide a treated
exhaust stream and discharging the treated exhaust stream into the
atmosphere, the combination of molecular sieves comprising only an oxide
(1) having a small pore crystalline molecular sieve or mixture of
molecular sieves having pores no larger than 8 membered rings and having
a mole ratio of at least 10 of (a) an oxide of a first tetravalent
element to (b) an oxide of a trivalent element, a pentavalent element, or
a second tetravalent element which is different from said first
tetravalent element or mixture thereof and an oxide (2) having a
crystalline molecular sieve or mixture of molecular sieves having pores
at least as large as 12 membered rings and having a mole ratio of at
least 10 of (a) an oxide of a first tetravalent element to (b) an oxide
of a trivalent element, a pentavalent element, or a second tetravalent
element which is different from said first tetravalent element or mixture
thereof.

15. A process for treating a cold-start engine exhaust gas stream
containing hydrocarbons and other pollutants comprising flowing said
engine exhaust gas stream over a combination of molecular sieves which
preferentially adsorbs the hydrocarbons over water to provide a first
exhaust stream, and flowing the first exhaust gas stream over a catalyst
to convert any residual hydrocarbons and other pollutants contained in
the first exhaust gas stream to innocuous products and provide a treated
exhaust stream and discharging the treated exhaust stream into the
atmosphere, the combination of molecular sieves comprising only an oxide
(1) having a small pore crystalline molecular sieve selected from the
group consisting of SSZ-16, SSZ-36, SSZ-39, SSZ-50, SSZ-52, SSZ-73 and
combinations thereof, and having a mole ratio of at least 10 of (a) an
oxide of a first tetravalent element to (b) an oxide of a trivalent
element, a pentavalent element, or a second tetravalent element which is
different from said first tetravalent element or mixture thereof and an
oxide (2) having a crystalline molecular sieve having pores at least as
large as 12 membered rings and selected from the group consisting of
SSZ-26, SSZ-33, SSZ-64, zeolite Beta, CIT-1, CIT-6, ITQ-4, and
combinations thereof, and having a mole ratio of at least 10 of (a) an
oxide of a first tetravalent element to (b) an oxide of a trivalent
element, a pentavalent element, or a second tetravalent element which is
different from said first tetravalent element or mixture thereof.

20. The process of claim 16, wherein the engine is an internal combustion
engine.

21. The process of claim 20 wherein the engine is fueled by a
hydrocarbonaceous fuel.

22. The process of claim 16, wherein the molecular sieve has deposited on
it a metal selected from the group consisting of platinum, palladium,
rhodium, ruthenium, and mixtures thereof.

23. The process of claim 22, wherein the metal is platinum.

24. The process of claim 22, wherein the metal is palladium.

25. The process of claim 22, wherein the metal is a mixture of platinum
and palladium.

26. The process of claim 15, wherein the cold-start engine exhaust gas
stream is contacted with the small pore crystalline molecular sieve prior
to contacting the crystalline molecular sieve having pores at least a
large as 12 membered rings.

Description:

[0001] This application is a continuation of U.S. application Ser. No.
11/961,776 filed Dec. 27, 2007, entitled "Treatment of Cold Start Engine
Exhaust" which claims benefit under 35 USC 119 of Provisional Application
60/882,081 filed Dec. 27, 2006, the contents of which are incorporated
herein by reference in their entirety.

FIELD OF THE INVENTION

[0002] The present invention relates to the treatment of cold start engine
exhaust using certain zeolites having different pore sizes.

BACKGROUND

[0003] Gaseous waste products resulting from the combustion of
hydrocarbonaceous fuels, such as gasoline and fuel oils, comprise carbon
monoxide, hydrocarbons and nitrogen oxides as products of combustion or
incomplete combustion, and pose a serious health problem with respect to
pollution of the atmosphere. While exhaust gases from other carbonaceous
fuel-burning sources, such as stationary engines, industrial furnaces,
etc., contribute substantially to air pollution, the exhaust gases from
automotive engines are a principal source of pollution. Because of these
health problem concerns, the Environmental Protection Agency (EPA) has
promulgated strict controls on the amounts of carbon monoxide,
hydrocarbons and nitrogen oxides which automobiles can emit. The
implementation of these controls has resulted in the use of catalytic
converters to reduce the amount of pollutants emitted from automobiles.

[0004] In order to achieve the simultaneous conversion of carbon monoxide,
hydrocarbon and nitrogen oxide pollutants, it has become the practice to
employ catalysts in conjunction with air-to-fuel ratio control means
which functions in response to a feedback signal from an oxygen sensor in
the engine exhaust system. Although these three component control
catalysts work quite well after they have reached operating temperature
of about 300° C., at lower temperatures they are not able to
convert substantial amounts of the pollutants. What this means is that
when an engine and in particular an automobile engine is started up, the
three component control catalyst is not able to convert the hydrocarbons
and other pollutants to innocuous compounds.

[0005] Adsorbent beds have been used to adsorb the hydrocarbons during the
cold start portion of the engine. Although the process typically will be
used with hydrocarbon fuels, adsorbent beds can also be used to treat
exhaust streams from alcohol fueled engines. The adsorbent bed is
typically placed immediately before the catalyst. Thus, the exhaust
stream is first flowed through the adsorbent bed and then through the
catalyst. The adsorbent bed preferentially adsorbs hydrocarbons over
water under the conditions present in the exhaust stream. After a certain
amount of time, the adsorbent bed has reached a temperature (typically
about 150° C.) at which the bed is no longer able to remove
hydrocarbons from the exhaust stream. That is, hydrocarbons are actually
desorbed from the adsorbent bed instead of being adsorbed. This
regenerates the adsorbent bed so that it can adsorb hydrocarbons during a
subsequent cold start.

[0006] The prior art reveals several references dealing with the use of
adsorbent beds to minimize hydrocarbon emissions during a cold start
engine operation. One such reference is U.S. Pat. No. 3,699,683 in which
an adsorbent bed is placed after both a reducing catalyst and an
oxidizing catalyst. The patentees disclose that when the exhaust gas
stream is below 200° C. the gas stream is flowed through the
reducing catalyst then through the oxidizing catalyst and finally through
the adsorbent bed, thereby adsorbing hydrocarbons on the adsorbent bed.
When the temperature goes above 200° C. the gas stream which is
discharged from the oxidation catalyst is divided into a major and minor
portion, the major portion being discharged directly into the atmosphere
and the minor portion passing through the adsorbent bed whereby unburned
hydrocarbon is desorbed and then flowing the resulting minor portion of
this exhaust stream containing the desorbed unburned hydrocarbons into
the engine where they are burned.

[0007] Another reference is U.S. Pat. No. 2,942,932 which teaches a
process for oxidizing carbon monoxide and hydrocarbons which are
contained in exhaust gas streams. The process disclosed in this patent
consists of flowing an exhaust stream which is below 800° F. into
an adsorption zone which adsorbs the carbon monoxide and hydrocarbons and
then passing the resultant stream from this adsorption zone into an
oxidation zone. When the temperature of the exhaust gas stream reaches
about 800° F. the exhaust stream is no longer passed through the
adsorption zone but is passed directly to the oxidation zone with the
addition of excess air.

[0009] Canadian Patent No. 1,205,980 discloses a method of reducing
exhaust emissions from an alcohol fueled automotive vehicle. This method
consists of directing the cool engine startup exhaust gas through a bed
of zeolite particles and then over an oxidation catalyst and then the gas
is discharged to the atmosphere. As the exhaust gas stream warms up it is
continuously passed over the adsorption bed and then over the oxidation
bed.

[0011] U.S. Pat. No. 5,603,216, issued Feb. 18, 1997 to Guile et al.,
discloses reducing the amount of hydrocarbons emitted during engine start
(cold start) using two zones in the exhaust system using the same or
different zeolite adsorber(s) in each zone. The zeolite(s) may be small
pore zeolite which adsorbs low molecular weight alkenes (ethylene and
propylene) and large pore zeolite which adsorb higher molecular weight
hydrocarbons (e.g., pentane). Disclosed examples of zeolites are ZSM-5,
Beta, gmelinite, mazzite, offretite, ZSM-12, ZSM-18, Berryllophosphate-H,
boggsite, SAPO-40, SAPO-41, Ultrastable Y, mordenite and combinations
thereof.

[0012] Elangovan et al., Journal of Physical Chemistry B, 108, 13059-13061
(2004) discloses the zeolite designated SSZ-33 (a zeolite having
intersecting 10 and 12 MR pores with a large void at the intersections)
for use as a hydrocarbon trap to reduce cold start emissions. The
performance of the SSZ-33 is compared to that of Beta, Y, mordenites and
ZSM-5 zeolites. SSZ-33 is said to have superior performance over Beta, Y,
mordenites or ZSM-5.

[0013] U.S. Patent Application Publication 2005/0166581, published Aug. 4,
2005 by Davis et al., discloses molecular sieves used as adsorbents in
hydrocarbon traps for engine exhaust. The method comprises contacting the
exhaust gas with molecular sieves having the CON topology (per the IZA).
The CON molecular sieve can be used by itself or can be used with another
adsorbent. Disclosed examples of CON molecular sieves are those
designated SSZ-33, SSZ-26, and CIT-1. ITQ-4 is also disclosed, but it is
believed ITQ-4 has the IFR topology, not the CON topology. Disclosed
examples of the other adsorbent are molecular sieves designated SSZ-23,
SSZ-31, SSZ-35, SSZ-41, SSZ-42, SSZ-43, SSZ-44, SSZ-45, SSZ-47, SSZ-48,
SSZ-53, SSZ-55, SSZ-57, SSZ-58, SSZ-59, SSZ-60, SSZ-63, SSZ-64, SSZ-65
and mixtures thereof.

SUMMARY OF THE INVENTION

[0014] This invention generally relates to a process for treating an
engine exhaust stream and in particular to a process for minimizing
emissions during the cold start operation of an engine. Accordingly, the
present invention provides a process for treating a cold-start engine
exhaust gas stream containing hydrocarbons and other pollutants
consisting of flowing said engine exhaust gas stream over a combination
of molecular sieves which preferentially adsorbs the hydrocarbons over
water to provide a first exhaust stream, and flowing the first exhaust
gas stream over a catalyst to convert any residual hydrocarbons and other
pollutants contained in the first exhaust gas stream to innocuous
products and provide a treated exhaust stream and discharging the treated
exhaust stream into the atmosphere, the combination of molecular sieves
comprising (1) a small pore crystalline molecular sieve or mixture of
molecular sieves having pores no larger than 8 membered rings ("8 MR")
selected from the group consisting of SSZ-13, SSZ-16, SSZ-36, SSZ-39,
SSZ-50, SSZ-52 and SSZ-73 and having a mole ratio of at least 10 of (a)
an oxide of a first tetravalent element to (b) an oxide of a trivalent
element, pentavalent element, second tetravalent element which is
different from said first tetravalent element or mixture thereof and (2)
a medium-large pore crystalline molecular sieve having pores at least as
large as 10 membered rings ("10 MR") selected from the group consisting
of SSZ-26, SSZ-33, SSZ-64, zeolite Beta, CIT-1, CIT-6 and ITQ-4 and
having a mole ratio of at least 10 of (a) an oxide of a first tetravalent
element to (b) an oxide of a trivalent element, pentavalent element,
second tetravalent element which is different from said first tetravalent
element or mixture thereof. The present invention also provides such a
process wherein oxides (1)(a) and (2)(a) are silicon oxide, and oxides
(1)(b) and (2)(b) are independently selected from aluminum oxide, gallium
oxide, iron oxide, boron oxide, titanium oxide, indium oxide, zinc oxide,
magnesium oxide, cobalt oxide and mixtures thereof. In one embodiment,
molecular sieve (1) is SSZ-13, SSZ-39 or mixtures thereof and molecular
sieve (2) is SSZ-26, SSZ-33, CIT-1, Beta, CIT-6 or mixtures thereof. In
another embodiment, molecular sieve (1), molecular sieve (2) or both
contain a metal selected from Cu, Ag, Au or mixtures thereof.

[0015] The present invention further provides such a process wherein the
engine is an internal combustion engine, including automobile engines,
which can be fueled by a hydrocarbonaceous fuel.

[0016] Also provided by the present invention is such a process wherein
the molecular sieve has deposited on it a metal selected from the group
consisting of platinum, palladium, rhodium, ruthenium, and mixtures
thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] FIGS. 1-3 illustrate data comparing the adsorptive properties of
zeolites with the adsorptive properties of mixtures of zeolites according
to the present invention.

[0021] As stated, this invention generally relates to a process for
treating an engine exhaust stream and in particular to a process for
minimizing emissions during the cold start operation of an engine. The
engine consists of any internal or external combustion engine which
generates an exhaust gas stream containing noxious components or
pollutants including unburned or thermally degraded hydrocarbons or
similar organics. Other noxious components usually present in the exhaust
gas include nitrogen oxides and carbon monoxide. The engine may be fueled
by a hydrocarbonaceous fuel. As used in this specification and in the
appended claims, the term "hydrocarbonaceous fuel" includes hydrocarbons,
alcohols and mixtures thereof. Examples of hydrocarbons which can be used
to fuel the engine are the mixtures of hydrocarbons which make up
gasoline or diesel fuel. The alcohols which may be used to fuel engines
include ethanol and methanol. Mixtures of alcohols and mixtures of
alcohols and hydrocarbons can also be used. The engine may be a jet
engine, gas turbine, internal combustion engine, such as an automobile,
truck or bus engine, a diesel engine or the like. The process is
particularly suited for hydrocarbon, alcohol, or hydrocarbon-alcohol
mixture, internal combustion engine mounted in an automobile. For
convenience the description will use hydrocarbon as the fuel to exemplify
the invention. The use of hydrocarbon in the subsequent description is
not to be construed as limiting the invention to hydrocarbon fueled
engines.

[0022] When the engine is started up, it produces a relatively high
concentration of hydrocarbons in the engine exhaust gas stream as well as
other pollutants. Pollutants will be used herein to collectively refer to
any unburned fuel components and combustion byproducts found in the
exhaust stream. For example, when the fuel is a hydrocarbon fuel,
hydrocarbons, nitrogen oxides, carbon monoxide and other combustion
byproducts will be found in the engine exhaust gas stream. The
temperature of this engine exhaust stream is relatively cool, generally
below 500° C. and typically in the range of 200° to
400° C. This engine exhaust stream has the above characteristics
during the initial period of engine operation, typically for the first 30
to 120 seconds after startup of a cold engine. The engine exhaust stream
will typically contain, by volume, about 500 to 1000 ppm hydrocarbons.

[0023] The engine exhaust gas stream which is to be treated is flowed over
a combination of molecular sieves in a first exhaust stream. The
combination of molecular sieves is described below. The first exhaust
stream which is discharged from the molecular sieve combination is now
flowed over a catalyst to convert the pollutants contained in the first
exhaust stream to innocuous components and provide a treated exhaust
stream which is discharged into the atmosphere. It is understood that
prior to discharge into the atmosphere, the treated exhaust stream may be
flowed through a muffler or other sound reduction apparatus well known in
the art.

[0024] The catalyst which is used to convert the pollutants to innocuous
components is usually referred to in the art as a three-component control
catalyst because it can simultaneously oxidize any residual hydrocarbons
present in the first exhaust stream to carbon dioxide and water, oxidize
any residual carbon monoxide to carbon dioxide and reduce any residual
nitric oxide to nitrogen and oxygen. In some cases the catalyst may not
be required to convert nitric oxide to nitrogen and oxygen, e.g., when an
alcohol is used as the fuel. In this case the catalyst is called an
oxidation catalyst. Because of the relatively low temperature of the
engine exhaust stream and the first exhaust stream, this catalyst does
not function at a very high efficiency, thereby necessitating the
molecular sieve adsorbent.

[0025] When the molecular sieve adsorbent reaches a sufficient
temperature, typically about 150-200° C., the pollutants which are
adsorbed in the molecular sieve(s) begin to desorb and are carried by the
first exhaust stream over the catalyst. At this point the catalyst has
reached its operating temperature and is therefore capable of fully
converting the pollutants to innocuous components.

[0026] The adsorption capacity of a molecular sieve depends on the size of
the hydrocarbon molecule (thus, its molecular weight and shape). For
example, when a molecular sieve having a smaller pore diameter (such as
an eight MR pore) is used, the hydrocarbons having higher molecular
weights (such as paraffins, olefins or aromatic compounds having at least
six carbon atoms) may not be adsorbed. To the contrary, when a molecular
sieve having a medium-large pore opening diameter (such as a twelve
and/or ten MR pore) is used, hydrocarbons having lower molecular weights
(such as methane, propane or propylene) is desorbed at a lower than
desired temperature, so that it is difficult to keep such a hydrocarbon
in the pores of the medium-large pore molecular sieve until the noble
metal becomes of sufficiently high temperature to be activated.

[0027] The molecular sieve adsorbent used in the present invention
comprises a combination or mixture of molecular sieves containing (1) a
molecular sieve or mixture of molecular sieves having pores no larger
than 8 membered rings ("8 MR") selected from the group consisting of
SSZ-13, SSZ-16, SSZ-36, SSZ-39, SSZ-50, SSZ-52 and SSZ-73 and having a
mole ratio of at least 10 of (a) an oxide of a first tetravalent element
to (b) an oxide of a trivalent element, pentavalent element, second
tetravalent element which is different from said first tetravalent
element or mixture thereof and (2) a medium-large pore crystalline
molecular sieve having pores at least as large as 10 membered rings ("10
MR") selected from the group consisting of SSZ-26, SSZ-33, SSZ-64,
zeolite Beta, CIT-1, CIT-6 and ITQ-4 and having a mole ratio of at least
10 of (a) an oxide of a first tetravalent element to (b) an oxide of a
trivalent element, pentavalent element, second tetravalent element which
is different from said first tetravalent element or mixture thereof.

[0028] The small pore molecular sieves of this invention have
(2-dimensional or 3-dimensional) intersecting channels. Examples of such
molecular sieves include the following, where the three letter structure
code, number of members in the pore ring(s) and channel configuration are
from the International Zeolite Association database:

[0036] The aforementioned patents cited to identify the small pore
molecular sieves useful in this invention are incorporated herein by
reference in their entirety.

[0037] The small pore molecular sieves listed above fulfill the criteria
for use in the present invention of having large micropore volumes and
high ratios of oxide(s) (1), e.g., silica, to oxide(s) (2), e.g., alumina
(referred to herein as "high silica" molecular sieves). These two
features distinguish these small pore molecular sieves from molecular
sieves used in the prior art wherein the small pore molecular sieves
contained large aluminum contents. This latter feature renders them much
more sensitive to collapse (sensitivity to steam under operating
conditions) than the high silica, small pore molecular sieves of this
invention.

[0038] The medium-large pore molecular sieves useful in this invention
have (2-dimensional or 3-dimensional) intersecting channels. The
medium-large pore molecular sieves should have a high internal pore
volume (e.g., a nitrogen adsorption capacity of about 0.18 cc/gm or
higher). Examples of such molecular sieves include the following, where
the three letter structure code, number of members in the pore ring(s)
and channel configuration are from the International Zeolite Association
database:

[0041] Molecular sieve designated SSZ-64, disclosed is U.S. Pat. No.
6,569,401, issued May 27, 2003 to Elomari. SSZ-64 is believed to have a
disordered structure with at least one 12 MR in the structure and a
micropore volume that exceeds 0.20 cc/g.

[0046] The aforementioned patents and literature article identifying the
medium-large pore molecular sieves used in the present invention are
incorporated herein by reference in their entirety.

[0047] The molecular sieves may comprise a framework heteroatom such as
Al, B, Ga, Fe, Zn, Mg, Co and mixtures thereof in addition to Si. The
molecular sieves may also contain a metal cation selected from rare
earth, Group 2 metals, Groups 8-10 metals and mixtures thereof, e.g., the
metal cation may be selected from Mn, Ca, Mg, Zn, Cd, Pt, Pd, Ni, Co, Ti,
Al, Sn, Fe, Co and mixtures thereof. The molecular sieves may also
contain a metal selected from Cu, Ag, Au and mixtures thereof. The
molecular sieves may also include other partial replacement atoms for Si
such as Ge. Techniques for replacing Si with Ge are known in the art
(see, for example, U.S. Pat. Nos. 4,910,006 and 4,963,337).

[0048] In one embodiment, the combination of small and medium-large pore
molecular sieves is a combination of (1) SSZ-13 and (2) SSZ-26, SSZ-33 or
mixtures thereof. In another embodiment, the combination of small and
medium-large pore molecular sieves is a combination of (1) SSZ-13 and (2)
SSZ-26.

[0049] The molecular sieves should be thermally stable to about
700° C., such as in the presence of steam. Steam can remove some
metals, such as aluminum from the framework of some zeolites, causing
their structure to collapse. Thus, it is important that the molecular
sieves used in the present invention be steam stable. If the molecular
sieve to be used does contain a metal, such as zinc, in the framework
which makes the molecular sieve unstable in a steam environment, that
metal can be replaced with an element that makes the molecular sieve
steam stable.

[0050] If the hydrocarbonaceous fuel undergoes incomplete combustion in
the engine, the exhaust gas can contain carbon dioxide and water. The
presence of water in the exhaust gas can make some molecular seizes
unstable. One way of stabilizing such molecular sieves is to increase the
amount of silicon oxide in the molecular sieve. In general, the higher
the silicon oxide content, the more hydrophobic the molecular sieve will
be, and the more stable it will be in the present of water vapor. Thus,
it may be desirable to partially or completely replace some metals (such
as zinc) in the framework of the molecular sieve with silicon to increase
hydrophobicity. In some cases, molecular sieves containing all-silicon
oxide may be desirable.

[0051] The small pore molecular sieve(s) and the medium-large pore
molecular sieve(s) of this invention are used in combination to treat a
cold-start engine exhaust gas stream. As used herein, the term
"combination" means that the cold-start engine exhaust gas stream is
contacted with both the small pore molecular sieve(s) of this invention
and the medium-large pore molecular sieve(s) of this invention prior to
the exhaust stream entering the catalytic converter. This can be
accomplished in a number of ways. For example, the "combination" may
comprise a mixture of the small pore and medium-large pore molecular
sieves in, e.g., a single bed. The small and large pore molecular sieves
may also be used in separated beds, or in a single bed comprising layers
of small pore and medium-large pore molecular sieves. The small and
medium-large pore molecular sieves may also be used in a single bed in
which the concentration of one, e.g., the small pore molecular sieve(s),
is high and the concentration of the medium-large pore molecular sieve(s)
is low (possibly as low as zero) at the upstream side of the bed. The
concentration of the small and medium-large pore molecular sieves then
gradually reverses in the downstream direction such that the
concentration of the, e.g., small pore molecular sieve(s) is low
(possibly as low as zero) and the concentration of the, e.g.,
medium-large pore molecular sieve(s) is high at the downstream end of the
bed. However, the small and medium-large pore molecular sieves are
conveniently disposed in separate, discrete beds. When used as such, it
is possible that, in the event one of the beds fouls, only the fouled bed
need be replaced, leaving the other bed intact.

[0052] The order in which the cold-start exhaust gas contacts the small
pore molecular(s) and the medium-large pore molecular sieve(s) may not be
critical. However, there may be advantages to contacting the cold start
exhaust gas with the small pore molecular sieve(s) prior to contact with
the medium-large pore molecular sieve(s). In this configuration, smaller
hydrocarbons (e.g., methane, propane and/or propylene) can be adsorbed by
the small pore molecular sieves, While the larger hydrocarbons bypass the
small pore molecular sieve(s) (because they are too large to fit in the
small pores) leaving the medium-large pore molecular sieve(s) free to
adsorb the larger hydrocarbons. The opposite configuration (i.e.,
medium-large pore molecular sieve(s) positioned upstream of the small
pore molecular sieve(s)) may be used as well. However, in this case,
there is a risk that the smaller hydrocarbons will fill the pores of the
medium-large pore molecular sieve(s) and block entry of the larger
hydrocarbons. In that event, the larger hydrocarbons may bypass the
pore-filled medium-large pore molecular sieve(s) as well as the
downstream small pore molecular sieve(s) (which are incapable of
adsorbing the larger hydrocarbons) and proceed to the catalytic converter
before the catalytic converter's temperature has risen to a temperature
sufficient to convert the larger hydrocarbons.

[0053] The particular configuration of the combination can take many
forms. For instance, the adsorbent bed can be conveniently employed in
particulate form or the adsorbent can be deposited onto a solid
monolithic carrier. When the particulate form is desired, the adsorbent
can be used in the form of powders, pills, pellets, granules, rings,
spheres, etc. In the employment of a monolithic form, it is usually most
convenient to employ the adsorbent as a thin film or coating deposited on
an inert carrier material which provides the structural support for the
adsorbent. The inert carrier material can be any refractory material such
as ceramic or metallic materials. It is desirable that the carrier
material be unreactive with the adsorbent and not be degraded by the gas
to which it is exposed. Examples of suitable ceramic materials include
sillimanite, petalite, cordierite, mullite, zircon, zircon mullite,
spondumene, alumina-titanate, etc. Examples of metallic materials which
serve as inert carrier material include metals and alloys as disclosed in
U.S. Pat. No. 3,920,583 which are oxidation resistant and are otherwise
capable of withstanding high temperatures.

[0054] The carrier material can be utilized in any rigid unitary
configuration which provides a plurality of pores or channels extending
in the direction of gas flow. Conveniently, the configuration may be a
honeycomb configuration. The honeycomb structure can be used
advantageously in either unitary form, or as an arrangement of multiple
modules. The honeycomb structure is usually oriented such that gas flow
is generally in the same direction as the cells or channels of the
honeycomb structure. For a more detailed discussion of monolithic
structures, refer to U.S. Pat. Nos. 3,785,998 and 3,767,453 which are
incorporated by reference herein.

[0055] The molecular sieve combination can be deposited onto the carrier
by any convenient way well known in the art. One convenient method
involves preparing a slurry using the molecular sieves which form the
combination (either together in a single slurry or separately in
different slurries) and coating the monolithic honeycomb carrier with the
slurry(ies). The slurry(ies) can be prepared by means known in the art
such as combining the appropriate amount of the molecular sieve(s) and a
binder with water. This resulting mixture(s) is then blended by using
means such as sonification, milling, etc. This slurry(ies) is used to
coat a monolithic honeycomb by dipping the honeycomb into the
slurry(ies), removing the excess slurry(ies) by draining or blowing out
the channels, and heating to about 100° C. If the desired loading
of molecular sieve combination is not achieved, the above process may be
repeated as many times as required to achieve the desired loading.

[0056] Instead of depositing the molecular sieve combination onto a
monolithic honeycomb structure, one can take the molecular sieve
combination and form it into a monolithic honeycomb structure by means
known in the art.

[0057] The adsorbent may optionally contain one or more catalytic metals
dispersed thereon. The metals which can be dispersed on the adsorbent are
the noble metals which consist of platinum, palladium, rhodium,
ruthenium, and mixtures thereof. The desired noble metal may be deposited
onto the adsorbent, which acts as a support, in any suitable manner well
known in the art. One example of a method of dispersing the noble metal
onto the adsorbent support involves impregnating the adsorbent support
with an aqueous solution of a decomposable compound of the desired noble
metal or metals, drying the adsorbent which has the noble metal compound
dispersed on it and then calcining in air at a temperature of about
400° to about 500° C. for a time of about 1 to about 4
hours. By decomposable compound is meant a compound which upon heating in
air gives the metal or metal oxide. Examples of the decomposable
compounds which can be used are set forth in U.S. Pat. No. 4,791,091
which is incorporated by reference. Preferred decomposable compounds are
chloroplatinic acid, rhodium trichloride, chloropalladic acid,
hexachloroiridate (IV) acid and hexachlororuthenate. It is preferable
that the noble metal be present in an amount ranging from about 0.01 to
about 4 weight percent of the adsorbent support. Specifically, in the
case of platinum and palladium the range is 0.1 to 4 weight percent,
while in the case of rhodium and ruthenium the range is from about 0.01
to 2 weight percent.

[0058] These catalytic metals are capable of oxidizing the hydrocarbon and
carbon monoxide and reducing the nitric oxide components to innocuous
products. Accordingly, the adsorbent bed can act both as an adsorbent and
as a catalyst.

[0059] The catalyst in the catalytic converter may be selected from any
three component control or oxidation catalyst well known in the art.
Examples of catalysts are those described in U.S. Pat. Nos. 4,528,279;
4,791,091; 4,760,044; 4,868,148; and 4,868,149, which are all
incorporated by reference. Preferred catalysts well known in the art are
those that contain platinum and rhodium and optionally palladium, while
oxidation catalysts usually do not contain rhodium. Oxidation catalysts
usually contain platinum and/or palladium metal. These catalysts may also
contain promoters and stabilizers such as barium, cerium, lanthanum,
nickel, and iron. The noble metals promoters and stabilizers are usually
deposited on a support such as alumina, silica, titania, zirconia,
alumino silicates, and mixtures thereof with alumina being preferred. The
catalyst can be conveniently employed in particulate form or the
catalytic composite can be deposited on a solid monolithic carrier with a
monolithic carrier being preferred. The particulate form and monolithic
form of the catalyst are prepared as described for the adsorbent above.

Example 1

[0060] The adsorption characteristics of zeolites were tested for their
effectiveness for adsorbing hydrocarbon materials normally found in
automobile exhaust streams. The SSZ-13 sample used in these tests had a
silica/alumina ratio of 15.6. The SSZ-33 sample used in these tests had a
silica/alumina ratio of 14.6.

[0061] Approximately 70 milligrams of a sample of zeolite SSZ-13 powder
were loaded in turn into a VTI Scientific Instruments GHP-FS Gravimetric
Sorption Analyzer. The sample preparation consisted of drying the sample
at 350° C. for 300 minutes (or until the sample weight changed by
less than 0.005% over a ten-minute period). The sample then was allowed
to equilibrate with methane gas at 30° C. and 500 torr pressure
for 30 minutes, or until the sample weight changed by less than 0.005%
over a fifteen-minute interval. The pressure was then increased at 500
torr intervals up to a maximum of 5000 torr, with the sample being
allowed to equilibrate with the methane gas after each pressure increase.
At each pressure, the methane adsorption amount was determined by the
weight change of the sample.

[0062] The method was repeated using a sample of zeolite SSZ-33 powder.

[0063] The method was repeated using a physical mixture of zeolite SSZ-13
powder and zeolite SSZ-33 powder.

[0064] The methane uptake by the following zeolites and uniform physical
mixtures of zeolites, reported in mmole/gram, is plotted in FIG. 1.

[0065] The data illustrated in FIG. 1 shows that methane uptake by SSZ-13
at 30° C. was greater on a weight basis than that of SSZ-33. The
1:1 mixture of SSZ-13: SSZ-33 (denoted 0.5SSZ13 in FIG. 1) was
intermediate between the methane uptake of the two zeolites alone.
However, the 4:1 and 3:1 mixtures of SSZ-13: SSZ-33 (denoted 0.8SSZ13 and
0.75SSZ13, respectively, in FIG. 1) are shown to have a methane uptake
which was equal to, or greater than that of SSZ-13 and SSZ-33 alone.

Example 2

[0066] Example 1 was repeated using ethane as an adsorbent. The ethane
uptake by the following zeolite samples, reported in mmole/gram, is
plotted in FIG. 2.